If installing SCS in database mode, it does require Enterprise MS SQL, per the users manual on page 34
"You can install the database on a computer running any of these Enterprise versions of SQL Server:
•Microsoft* SQL Server* 2008 x32/x64 (SP1)
•Microsoft SQL Server 2008 R2 x64
•Microsoft SQL Server 2005 x32 (SP4) "
I've manage to diagnose the problem.
Correct me if i'm wrong but RCS installs itself as a 32 bits program in C:\Program files (x86).
Since my computer runs on Windows 7 x64 with 24Gb of RAM i installed SQL Server 2008 Express R2 x64.
Then i suspect that RCS tries to locate a 32 bits version of SQL Server.
I've resolved the issue by uninstalling SQL Server x64 and install SQL Server 2008 Express x86 and then RCS found the SQL Server and finished installing.
Then, my question becomes why doesn't RCS installs itself in 64 bits, and there are only one software to download?
The error is most likely an issue with your WMI permission for the RCS. Inspect you SCS Installation guide for details. If you seem to have your permissions in order, open the RCS WMI permission to temporarily include "Everyone", restart the rcs service and see if the permissions are now in order.
Let me know what happens